The present invention relates to shoe press units used, for example, in papermaking for pressing a paper web. The invention relates more particularly to a method and a shoe press unit in which excess lubricating oil that is expelled from between a pressing surface of a press shoe and a flexible belt is captured and evacuated from the shoe press unit.
A shoe press unit typically comprises a support beam, a shoe element movably supported on the beam, a pressing unit arranged between the beam and the shoe element for urging the shoe element away from the beam and toward a counter element such as a counter roll, and a flexible belt that is arranged to slide over the pressing surface of the shoe element. To reduce friction between the belt and the shoe element and thereby reduce the frictional heating of the belt, it is common to supply a lubricating oil between the pressing surface of the shoe element and the belt. The oil both lubricates and cools the belt and the pressing surface. Excess oil is expelled from between the belt and the pressing surface as a result of the pressure exerted in the nip between the shoe element and the counter element. The excess oil is expelled from an upstream edge region of the pressing surface, and is then evacuated from the shoe press unit by an oil evacuation arrangement.
U.S. Pat. No. 5,935,385 discloses a shoe press unit having an oil evacuation arrangement in which an inlet opening of the oil evacuation arrangement is arranged on the beam at a distance from the shoe element. Therefore, the oil evacuation arrangement does not move with the shoe element. The inlet opening is so located that most or all of the initial kinetic energy of the excess oil exiting from between the belt and pressing surface is lost before the excess oil passes through the inlet opening. Thus, this kinetic energy of the oil is not available to assist in evacuating the oil. Another disadvantage of the oil evacuation arrangement is that it does not prevent the excess oil from flowing in various directions within the shoe press unit, and hence the oil tends to accumulate in the shoe press unit. The accumulated oil tends to mix with air, which makes evacuation of the oil more difficult and also requires a subsequent processing of the evacuated oil to separate the air from the oil prior to reusing the oil. The accumulated oil, which is relatively hot because of the heat transfer from the belt to the oil, also tends to conduct heat to other parts of the shoe press unit before it is evacuated, which results in an undesirable temperature increase inside the shoe press unit. Moreover, it is disadvantageous to have an accumulation of oil in the shoe press unit because this requires an increased power consumption. Finally, constructing the oil evacuation arrangement as an integral part of the shoe element requires relatively costly manufacturing methods.
The present invention addresses the above and other needs by providing a method and a shoe press unit in which an oil evacuation arrangement is affixed to the shoe element proximate an upstream edge region of its pressing surface, such that the shoe element and oil evacuation arrangement move together as a unit. The oil evacuation arrangement includes an evacuation duct for evacuating the excess oil expelled from between the belt and the shoe element. The evacuation duct is coupled to an outlet pipe for the evacuated oil within the shoe press unit. The evacuation duct is fixed relative to the shoe element and the outlet pipe is fixed relative to the beam, and the evacuation duct is movably connected to the outlet pipe such that the duct can move relative to the outlet pipe in at least the pressing direction along which the shoe element is moved by the pressing unit.
In accordance with a preferred embodiment of the invention, the oil evacuation arrangement comprises a container having a bottom and a plurality of wall elements upstanding from the bottom. The excess oil is squirted out from between the belt and pressing surface through an inlet opening of the container. The evacuation duct preferably includes a substantially rigid tubular member that is affixed in the container. In one embodiment, the tubular member extends through a through-hole formed in the beam and connects with the outlet pipe arranged in the interior of the beam.
The duct is coupled to the outlet pipe in one embodiment via a flexible sealing device that accommodates relative movement between the duct and outlet pipe in at least the pressing direction, and preferably also accommodates lateral movement of the duct that can arise for example from thermal expansion or lateral movement of the shoe element. In one embodiment, the sealing device comprises a bellows formed of an elastomeric material such as rubber. In another embodiment, the duct is coupled to the outlet pipe via a pair of tubular members one of which is slidably and sealingly received in the other. One of the tubular members can be fixedly connected to the container on the shoe element, and the other tubular member can be fixedly coupled with the outlet pipe. In yet another embodiment, the tubular member fixed to the container is slidably received in a sealing manner in an opening formed through a wall of the outlet pipe. The tubular member is sealed relative to the outlet pipe by one or more seals arranged at the opening in the outlet pipe.
The shoe press unit in a preferred embodiment comprises a closed shoe press unit, and the interior of the shoe press unit has an overpressure relative to the pressure outside the shoe press unit of 10-500 mbar. More preferably, the interior overpressure is below 200 mbar, and most preferably is below 50 mbar. The outlet pipe can be connected to a vacuum source outside the shoe press unit to facilitate the evacuation of oil.